The present invention relates generally to injection molding apparatus and, more particularly, to a needle valve gate assembly used in an injection molding apparatus to prevent the creation of weld lines and similar defects in a molded article produced by the apparatus, and particularly, to prevent the creation of weld lines in polystyrene parisons formed by the apparatus and used as a preform in the blowing of plastic bottles.
Containers constructed from thermoplastic material have become widely used in the beverage industry, competing with traditional metal and glass containers. Attractive features of plastic containers include resistance to rupture; transparency, allowing viewing of the bottled contents; and light weight, reducing shipping costs. However, a drawback of thermoplastic beverage bottles has been the relatively high cost of the plastic materials which are presently used to produce such bottles. Polystyrene is a low cost material which is readily available as a by-product from the formation of more expensive plastic materials. However, there are presently no polystyrene containers in commercial use as containers for pressurized beverages. Polystyrene bottles to be economically feasible must be capable of being produced rapidly and, as a finished product must have sufficient strength to withstand rupture under normal use and to contain beverages under pressure without significant distortion. The physical characteristics of polystyrene make use of this plastic in pressurized containers which may be economically mass produced quite different.
The formation of a plastic bottle usually includes the step of producing a tube shaped preform or "parison" in an injection molding operation and thereafter allowing the parison to cool to a some degree. Next, the parison is placed in a pressure chamber wherein a pressure differential and usually heat are applied to the container to cause it to expand into the shape of the finished plastic bottle. The time period for each of these operations is preferably kept as short as possible to increase the speed of production.
However, it has been found that since polystyrene has inferior strength characteristics to most plastics used in bottles, the wall thickness of a polystyrene bottle must generally be thicker than with other plastics. Thus, the parison used to produce the blow molded polystyrene bottle must also be relatively thick. The increased thickness of the polystyrene bottle causes a significant problem from a production cost standpoint. Since the injection molded parison is thicker than most parisons used in the formation of plastic bottles, the cooling time of the polystyrene parison must be longer in order to prevent crystalization or similar phenomena caused by rapid cooling which adversely affects the strength and other characteristics of the completed parison. Since the cooling time of the parison is a critical economic parameter, it is desirable to heat the polystyrene to the lowest temperature possible which will still allow proper polymer flow through the injection mold. However, if the temperature of the molten polystyrene in the injection mold becomes too low, certain undesirable features, such as weld lines occur in the parison being formed and/or the later formed blow molded bottle.
Thus, lowering of the temperature of the molten plastic during injection molding without producing adverse physical affects in the parison being formed is critical to the production cost of polystyrene bottles.
The use of injection molds for forming plastic articles is well known. A typical injection mold includes a hopper apparatus which is filled with a plurality of plastic pellets. The hopper apparatus dispenses the particles into an auger or screw device which has a progressively narrower pitch extending outwardly from the hopper device which causes the "solid" plastic pellets to be heated and compressed as they are moved away from the hopper. At the end of the auger device, distal the hopper, the plastic pellets have been sufficiently compressed and heated by friction to assume a liquid state. A high temperature, high pressure, high viscosity molten polymer is thus dispensed from the auger orifice and is directed through suitable valve and conduit means to a mold cavity which defines the shape of an article to be produced. One conventional manner for controlling the flow of molten polymer to the mold cavity includes passing the molten polymer leaving the auger device through a three way valve. In one position, the liquid plastic is directed from the auger through the valve into an injection pot having a moving piston therein which is withdrawn to cause the pot to receive molten polymer from the auger. When the injection pot is sufficiently filled with molten polymer, the three way valve is shifted such that the injection pot is placed in communication, through a suitable conduit and valve arrangement, with the mold cavity of the apparatus. A needle valve gate assembly is generally provided in the conduit between the mold cavity and the injection pot in order to precisely control the flow of molten polymer into the cavity. When it is desired inject the mold cavity with molten polymer, the needle valve is withdrawn from the orifice of the conduit and the piston portion of the injection pot is urged in a direction to expel the polymer therefrom. Thereafter polymer flows out of the injection pot through the three way valve and connecting conduit, and the needle valve assembly into the mold cavity.
It is conventional for a gate assembly used in this type of apparatus to comprise a short, straight length, conduit portion which terminates in a conical shaped orifice in which the needle valve seats. Such a conventional gate assembly also comprises a needle valve having an elongate needle with a central longitudinal axis positioned coaxially with the central longitudinal axis of the short, straight length conduit portion. In order to close the gate, a tip portion of the needle is urged axially into closing relationship with the orifice portion of the conduit. In order to open the gate, the needle is withdrawn a short distance axially thus allowing a flow of polymer around the needle and out through the orifice. The needle generally extends axially rearwardly until passing out of the short length conduit portion through a rear wall seal portion thereof. Thus, the polymer is forced to "split" and flow around the needle through the entire short, straight length portion of the conduit immediately prior to passing out the discharge orifice of the conduit.
It is a discovery of the present applicants that due to the elastic memory of the molten polymer, this splitting of the polymer flow to flow around the needle, at lower temperature ranges, causes imperfections in the article being molded which are manifested in the form of "weld lines". Weld lines refer to relatively narrow lines formed in the plastic which indicate the presence of a weakened zone in the plastic article being formed. Until applicants discovery, the manner of reducing weld lines in a polystyrene forming process was to increase the operating temperature of the polymer. However, in a commercial setting, this led to one of two undesirable results. An article having inferior strength characteristics is formed if cooled in the same time period as articles formed at lower temperatures. And, if the article is allowed to cool slowly, the increased cooling time substantially increased the production time and thus increased the fabrication costs of the article.
It is applicants' discovery that the construction of the needle valve that is currently in use in the industry contributes significantly to the formation of weld lines and associated problems in injection molded products. The needle valve of applicants' invention allows completely unrestricted flow through the end portion of the conduit which passes liquid polymer to the mold cavity when the gate assembly is open and thus eliminates or greatly reduces the occurrence of weld lines and other associated problems over a considerable operating range as compared to the results when a conventional needle valve is used.